A. Field of the Invention
This invention relates to the field of automatic acquisition of data from a plurality of thermocouples.
B. Prior Art
Known data acquisition systems have many thermocouples located at various points in, for example, power generating stations and chemical processing plants. The thermocouples are continuously monitored and data from each are recorded at desired intervals. Thermocouples having dissimilar metals bonded together commonly fail by becoming open circuited. Since the output of a normal thermocouple junction is a very small voltage value corresponding to the temperature difference with respect to a reference junction the output from a failed device would be indistinguishable from that of a normal device, especially since an open circuit at the input of an instrumentation amplifier would permit small offset and leakage currents to provide a small random voltage difference.
In some prior systems, thermocouples have been tested for an open condition by connecting them to a current source and measuring the resultant current flow or voltage drop. One such method which has been used successfully involves the addition of a common dry cell battery to each thermocouple circuit so a micro-ampere of current continuously flows through the thermocouple producing a negligible voltage drop if it is normal, but creating a terminal voltage of about one volt if it were open. The disadvantage of course is the need to replace, annually at least, hundreds of batteries in a typical multipoint system. Equivalent systems have previously been used in which the test current is provided by an instrumentation power source in lieu of batteries. Such prior systems however have introduced other drawbacks such as some cross-coupling of signals or the creation of error voltages by the flow of current in the test circuit due to the common-mode voltage present at the thermocouple.
In other data acquisition systems, each of the multitude of thermocouples, has an associated pair or resistors and a shunt capacitor designed to attenuate normal mode noise at the input to the amplifier. These filter components render ineffective testing means which introduce from the amplifier end, brief current pulses to the thermocouple loop because such pulses would be bypassed by the filter capacitor regardless of the state of the thermocouple. Furthermore, since the data acquisition system comprises assemblies of thermocouple filters and relays, connection could not be established directly to the thermocouple beyond the resistors for the purpose of detecting abnormal voltage without incurring the great expense of adding relays for each thermocouple point.
Accordingly, an object of this invention is to provide means for quickly charging the filter capacitor associated with each thermocouple to a recognizably high voltage and isolating such circuit from all other circuits in the system during this event. This voltage will remain on the capacitor if the associated thermocouple is open circuited and will give an indication of an open thermocouple.
A further object of this invention is to control the rates at which said charging current is turned on and off to minimize or eliminate errors caused by inductive coupling with other circuits.